A common configuration of rolling mill has four or more rolls mounted in a vertical plane with two smaller diameter work rolls supported between larger diameter back-up rolls. Such mills may operate in isolation or in tandem with other similar mill stands.
A particular problem of importance in mill control arises from out-of roundness in one or more of the rolls which produces cyclic variations in the gap between the rolls. These variations in gap cause corresponding changes in roll separating force, metal velocities and, most importantly, in the thickness of the product issuing from between the rolls.
Control of output product thickness is usually effected by changing the relative gap between the work rolls by means of a motor driven screw or hydraulic cylinder acting on the back-up roll bearings. Usually the bearing position is measured with respect to the support frame (the so-called "rollgap position"). The separation of the work rolls cannot be directly measured by the roll gap position because of significant elastic deformations in the mill stand components.
It is conventional practice to provide a rolling mill stand with a transducer for measuring the total deformation force applied to the workpiece and another for measuring the roll gap position.
Furthermore, it is often desirable to install a thickness measuring gauge after the stand to monitor the operation of the process and the effectiveness of any thickness control system which may be installed.
It is well known to those skilled in this art that the dynamic response of a feedback control system is deleteriously affected if a time delay occurs between the creation of a change and measurement of the change and for this reason techniques have been developed for estimating the rolled strip thickness from a knowledge of the nominal gap between the rolls and the change in this gap due to elastic deformations which are calculated as a function of measured force and nominal material width. This "instantaneous" estimate of product thickness can be used for feedback control to the stand on which measurements were obtained or for feedforward control to downstream stands. Major benefits are gained by use of this technique if the rollgap adjusting mechanism has a response time which is significantly less than the time delay to the measured thickness obtained downstream.
A major drawback of the feedback and feedforward control techniques described above is that if the mill work rolls and backup rolls are not perfectly round, the measured rollgap position is not equal to the true roll gap position, and eccentricity induced signal components appear in the force and thickness measurements. These lead to an incorrect "estimated thickness" which results in the control systems correcting non-existent errors, thereby creating worse product thickness deviations than are likely to arise with no control.
Numerous techniques have been proposed for overcoming this problem including tuned filters, adjustable deadbands, the addition of force control systems and direct measurement of the eccentricity effects as the rolls rotate with subsequent subtraction to cancel their effect. The latter technique has been shown to have some beneficial results but suffers from the need to install eccentricity measuring equipment on the rolls producing the eccentricity component in the transducer signals.
Normally the back-up rolls are the major source of the eccentricity signal components although the work rolls or other, intermediate rolls, may also contribute.
It is an object of the present invention to provide a simple and effective method for eliminating the effect of multiple, superimposed cyclic variations caused by the individual roll eccentricity signals. The method proposed is capable of operation without direct measurement of the angular position of all the rolls. However, if such information is available, it may be used in the proposed method to obtain further benefits. Accurate, angular speed or position information is readily available for the driven rolls, usually the work rolls in a four-high configuration. The angular position measurement is preferred to an integrated speed measurement because of its inherently greater accuracy. These signals and a knowledge of all the roll diameters is sufficient to implement the proposed method of roll eccentricity control.